Prostaglandins have been implicated in the modulation of the hydroosmotic effect of antidiuretic hormone (ADH) in the mammalian kidney. This study investigated the physiological role of renal prostaglandins in 5 normal female subjects. The subjects underwent 2 infusions of ADH, before and after prostaglandin inhibition by oral adminstration of indomethacin. It could be shown that urine volume decreased while the remaining renal functional parameters (glomerular filtration rate, renal plasma flow, sodium excretion and urinary osmolality) were not affected by prostaglandin synthesis blockade. Urinary cyclic AMP excretion, however, was significantly decreased on indomethacin treatment indicating that prostaglandins may directly compete with ADH at the tubular receptor.